Three new alkaline-earth metal uranyl selenites, Ca[(UO 2 ) (SeO 3 ) 2 ] (1), Sr[(UO 2 )(SeO 3 ) 2 ] . 2H 2 O (2), and Ba[(UO 2 ) (SeO 3 ) 2 ] (3), have been prepared from the reactions of CaCO 3 and Ca(OH) 2 , SrCl 2 and Sr(OH) 2 , or BaCl 2 and Ba(OH) 2 with UO 3 and SeO 2 under mild hydrothermal conditions. Singlecrystal X-ray diffraction experiments reveal that the structures of 1-3 differ in both connectivity and dimensionality even though all contain the same fundamental building unit, namely [UO 2 (SeO 3 ) 4 ]. This polyhedron consists of a linear uranyl unit that is bound by one chelating and three bridging selenite anions creating a pentagonal bipyramidal environment around the U(VI) center. The crystal structure of 1 contains onedimensional ribbons where the edges are terminated by monodentate selenite anions. The interior of the ribbons are constructed from edge-sharing pentagonal bipyramidal UO 7 units. The structure of 2 is also one-dimensional; however, here there are chains of edge-sharing pentagonal bipyramidal UO 7 dimers that are connected by bridging selenite anions. Ba[(UO 2 )(SeO 3 ) 2 ] (3) is two-dimensional, and the highly ruffled anionic sheets present in this structure are formed from both bridging and chelating/bridging selenite anions bound to uranyl moieties. The anionic substructures in 1-3 are separated by Ca 2+ , Sr 2+ , or Ba 2+ cations. Crystallographic data (193 K, MoKa, k=0.71073): 1, triclinic, space group P % 1; a=5.5502(6) ( A, b=6.6415(7) ( A, c=11.013(1) ( A, a=104.055(2)1, b=93.342(2)1, c=110.589(2)1, Z=2, R(F)=4.56% for 100 parameters with 1530 reflections with I>2r(I); 2, triclinic, space group P % 1; a=7.0545(5) ( A, b=7.4656(5) ( A, c=10.0484(6) ( A, a=106.995(1)1, b=108.028(1)1, c=98.875(1)1, Z=2, R(F)= 2.43% for 128 parameters with 2187 reflections with I>2r(I); 3, monoclinic, space group P2 1 =c; a=7.3067(6) ( A, b=8.1239(7) ( A, c=13.651(1) ( A, b=100.375(2)1, Z=4, R(F)=4.31% for 105 parameters with 1824 reflections with I>2r(I). # 2002 Elsevier Science (USA)